The present invention relates generally to orifice pulse tube refrigerators, and, more particularly, to orifice pulse tube refrigerators with reduced Rayleigh streaming in the pulse tube.
Orifice pulse tube refrigeration is the most rapidly developing field of cryogenic refrigeration today. The high efficiency of a Stirling-based thermodynamic cycle, the lack of moving parts at cryogenic temperature, and the lack of small, easily plugged orifices at cryogenic temperature combine to make this new technology inexpensive and reliable. Furthermore, orifice pulse tube refrigerators can be driven by thermoacoustic heat engines, creating for the first time cryogenic refrigeration with no moving parts. Background information about orifice pulse tube refrigerators is given, for example, by R. Radebaugh, xe2x80x9cA review of pulse tube refrigeration,xe2x80x9d pages 1191-1205 in Adv. Cryogenic Eng., Volume 35 (1990), and in R. Radebaugh, xe2x80x9cAdvances in Cryocoolers,xe2x80x9d 1997, pages 33-44 in the Proceedings of the Sixteenth International Cryogenic Engineering Conference/international Cryogenic Materials Conference (ICEC16/ICMC), edited by T. Haruyama et al., (Elsevier, Oxford, 1997), all incorporated herein by reference.
A prior art orifice pulse tube refrigerator is shown schematically in FIG. 1A. One of the key parameters in an operational orifice pulse tube refrigerator is the temporal phase difference between oscillating pressure and oscillating velocity. The reference convention used herein is that x is the distance from the driver along the axis of the refrigerator, that positive velocity is velocity in the positive x direction, and that xcex8 is the temporal phase angle by which oscillating pressure leads oscillating velocity. This temporal phase xcex8 is also called the phase of the complex acoustic impedance Z. It is well known that xcex8 is a function of x, due for example, to the compressibility of the gas in various portions of the refrigerator.
Relative magnitudes and phases are conventionally displayed in a phasor diagram, such as shown in FIG. 1B. FIG. 1B illustrates, for example, the viscous pressure drop across regenerator 12, which manifests itself as the difference between the pressure phasor P1,driver at driver 10 and the pressure phasor P1,pulse tube in pulse tube 18. The difference between the volume flow rate phasor U1,ambient heat exchanger at ambient heat exchanger 28 and the volume flow rate phasor U1,cold at cold heat exchanger 26 is due to the compressibility of the gas in pulse tube 18. In FIG. 1B, P1,ambient heat exchanger can be assumed nearly identical to P1,pulse tube, SO xcex8ambient heat exchanger is the angle by which P1,pulse tube leads U1,ambient heat exchanger, which is approximately 50 degrees in FIG. 1B. This is a typical value for an orifice pulse tube refrigerator with an inertial impedance (xe2x80x9cinertancexe2x80x9d).
Continuing to refer to FIGS. 1A and 1B, it is well known that the entire phase distribution xcex8(x) throughout an orifice pulse tube refrigerator, and, in particular, in regenerator 12, can be controlled by means of inertance 14 and flow resistances 16, 24 in an acoustic impedance network atop pulse tube 18 of the orifice pulse tube refrigerator. An early published reference to this use of inertance was by S. W. Zhu et al., xe2x80x9cPhase shift effect of the long neck tube for the pulse tube refrigerator,xe2x80x9d in the Proceedings of Cryocoolers 9, held June 1996 in New Hampshire. An adjustable version of such an acoustic impedance network with inertance is described by D. L. Gardner et al., xe2x80x9cUse of inertance in orifice pulse tube refrigerators,xe2x80x9d Cryogenics, Volume 37, pages 117-121 (1997) and G. W. Swift et al., xe2x80x9cPulse Tube Refrigerator With Variable Phase Shift,xe2x80x9d U.S. Pat. No. 6,021,643, Feb. 8, 2000, all incorporated herein by reference. In the ""643 patent, a variable acoustic impedance network, as shown atop pulse tube 18 in FIG. 1A, is described, comprising an inertance tube 14, a compliance volume 22, and two adjustable flow resistance valves 16,24.
FIG. 2, which is a reproduction of FIG. 7 from the ""643 patent, shows the broad range of xcex8ambient heat exchanger accessible by this method. The points on FIG. 2 show some typical values of acoustic impedance Z at the top of the pulse tube, experimentally accessed by adjusting the two valves 16, 24; all points between these points are also accessible. Absent viscous effects in inertance 14, all values of Z between the two horizontal dashed lines would be accessible, and the experimental reality is not far from that ideal. The values of xcex8 represented by these points range from about zero to 80 degrees (the angle between the horizontal axis and a line from the origin to a given point).
The three large circles on FIG. 2 are contours of constant power dissipation in the acoustic impedance network 14, 16, 22, 24, and, hence, of constant gross cooling power at cold heat exchanger 26. Then, an operating point for the orifice pulse tube refrigerator is uniquely defined by, and is often chosen by, selecting a gross cooling power, i.e., at which circle one wants to operate, and a value of xcex8. The actual net refrigerating power is the gross cooling power minus the sum of heat leaks to cold heat exchanger 26. Imperfect operation of regenerator 12 and imperfect operation of pulse tube 18 are two sources of potentially large heat leaks, but proper design can minimize these. Efficient refrigeration also requires little viscous dissipation in regenerator 12.
It is well known that refrigeration occurs only if xcex8 lies between plus 90 degrees and minus 90 degrees in regenerator 12, and that both regenerator heat leak and viscous dissipation are minimized by keeping xcex8 as close to zero degrees as possible throughout regenerator 12. In a cryogenic orifice pulse tube refrigerator, typically xcex8 is between zero and minus 45 degrees at the ambient end of the regenerator, passes through zero somewhere within the regenerator, and is positive and less than 45 degrees at the cold end of the regenerator. However, the sensitivity of regenerator efficiency to the exact values of xcex8(x) is not too strong, and a regenerator with xcex8(x) shifted by 10 or even 20 degrees from the optimal values may not have a noticeable loss in efficiency with respect to either viscous dissipation or heat leak.
The temporal phase xcex8 also plays an important role in the efficiency of the pulse tube of the orifice pulse tube refrigerator. Pulse tubes are susceptible to an internal, toroidal steady convection, called Rayleigh streaming, that is superimposed upon the desired oscillatory motion. Rayleigh streaming reduces the efficiency of orifice pulse tube refrigerators because the streaming convects heat from ambient heat exchanger 28 atop pulse tube 18 to cold heat exchanger 26 at the bottom of pulse tube 18, thereby reducing the cooling power of the orifice pulse tube refrigerator. Rayleigh streaming is caused by boundary-layer processes at the side walls of the pulse tube, which are controlled by various parameters including phase angle xcex8, the taper angle of the pulse tube, and properties of the working gas, as described by J. R. Olson et al., xe2x80x9cAcoustic streaming in pulse tube refrigerators: Tapered pulse tubes,xe2x80x9d Cryogenics, Volume 37, pages 769-776 (1997) and G. W. Swift et al., xe2x80x9cTapered pulse tube for pulse tube refrigerators,xe2x80x9d U.S. Pat. No. 5,953,920, Sep. 21, 1999, all incorporated herein by reference. All other variables being fixed, there is at most one value of xcex8 that stops Rayleigh streaming.
Rayleigh streaming is extremely sensitive to the value of xcex8, as shown in FIG. 3, from G. W. Swift et al., xe2x80x9cPerformance of a tapered pulse tube,xe2x80x9d pages 315-320 in Cryocoolers 10, edited by R. G. Ross Jr. (Kluwer Academic/Plenum Publishers, 1999), incorporated herein by reference. This experimental evidence shows that a 3 degree change in xcex8 away from the optimum value can cause enough Rayleigh streaming to consume 10% of the gross cooling power of the orifice pulse tube refrigerator. A 3 degree change in xcex8 is small enough that it would have no significant effect on the regenerator efficiency.
The temporal phase xcex8 can be adjusted, as described in the ""643 patent, but there is no need for such a large range of adjustability when nominally identical orifice pulse tube refrigerators are mass produced for nominally identical applications. In such a circumstance, an acoustic impedance network with geometrically fixed components would be much cheaper than the high-pressure bellows-sealed valves described in Swift et al., supra. For automated control, expensive high-torque valve actuators may also be needed to adjust the resistances automatically. However, it is often necessary to provide fine-tuning adjustment of the acoustic impedance network, because of the sensitivity of Rayleigh streaming to the conditions of operation. Even nominally identical orifice pulse tube refrigerators that are mass produced for nominally identical applications may suffer from minor unit-to-unit construction variations or from diurnal and seasonal variations in ambient temperature.
Hence, it is desirable to provide fine-tuning adjustments to the value of xcex8 in the pulse tubes of orifice pulse tube refrigerators. It is further desirable to provide for the fine-tuning adjustments with inexpensive hardware.
Various advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The present invention includes an orifice pulse tube refrigerator having flow resistance, compliance, and inertance components connected to a pulse tube for establishing a phase relationship between oscillating pressure and oscillating velocity in the pulse tube. A temperature regulating system heats or cools a working gas in at least one of the flow resistance and inertance components. A temperature control system is connected to the temperature regulating system for controlling the temperature of the working gas in the at least one of the flow resistance and inertance components and maintains a control temperature that is indicative of a desired temporal phase relationship.